Capacitor and method of making same

ABSTRACT

A capacitor having a dielectric consisting of a glass layer with an alkali metal oxide content of at most 2 wt % and a thickness of at most 50 μm is provided. The capacitor includes at least two metal layers which are separated by the glass layer. The glass layer is preferably produced by a down-draw method or by an overflow down-draw fusion method.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional of Ser. No. 13/205,832, filed on Aug.9, 2011, which is a continuation of International Patent ApplicationPCT/EP2010/000826, filed on Feb. 10, 2010 designating the U.S., whichInternational Patent Application has been published in the Germanlanguage and claims the benefit from German patent application 10 2009008 292.1, filed on Feb. 10, 2009. The entire content of all of thesepriority applications are incorporated in their entirety herein byreference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a capacitor having a glass dielectric and to amethod for the production thereof.

2. Description of Related Art

The constantly increasing need to store energy for times frommicroseconds to days with a large amount of electrical energy isnecessitating materials having special dielectric properties.

Polypropylene film capacitors are generally used as high-powercapacitors in the prior art. They can be rolled, and thedielectric—polypropylene—can be produced in the form of a very thinfilm. However, the dielectric breakdown voltage of approximately 1 MV/cmlimits the density with which energy can be stored.

Electrolyte capacitors can also be brought to very high storagedensities. For instance, Jeol in Japan has reported double-layerelectrolyte capacitors with storage densities of the order of 20 Wh/l(JP 11288852 A2).

Such electrolytes, however, are in general chemically reactive andenvironmentally harmful, and in some cases may even be explosive.

Ceramic capacitors are furthermore known which use ceramics asdielectric, for example capacitors comprising ferroelectric phases.However, the residual porosity of ceramics limits their dielectricbreakdown field strength.

Batteries, for instance lithium ion batteries, currently give thegreatest storage density of up to 800 Wh/l. Batteries, however, have alimited lifetime and, in contrast to capacitors, have charging times ofup to several hours.

Glasses as dielectric for capacitors are furthermore known in the priorart. Glass capacitors have been described since the 1940s. They arewidely used in electronic applications for satellites and space flight,and can work in very aggressive environmental conditions. It is alsoknown that the dielectric breakdown field strength of glasses isgenerally greater than that of the corresponding crystal systems. A.Hippel, R. J. Maurer, Phys. Rev., 59, 820 (1941) describe thisphenomenon by comparing the breakdown field strength of quartz glasswith quartz crystal. The authors indicate a breakdown field strength ofthe order of 7 MV/cm for quartz glass.

However, the production of capacitors from quartz glass would be anextraordinarily expensive and time-consuming process.

BRIEF SUMMARY OF THE INVENTION

It is a first object of the invention to disclose an improved capacitorhaving a high reliability.

It is a second object of the invention to disclose an improved capacitorhaving a high power density.

It is a third object of the invention to disclose an improved capacitorhaving a glass as dielectric.

It is a forth object of the invention to disclose an improved capacitorhaving an improved breakdown strength.

It is a fifth object of the invention to disclose a method of makingsuch an improved capacitor.

According to the invention these and other objects are achieved by acapacitor comprising at least two metal layers which are separated by adielectric configured as a glass layer with a fire-polished surface, theglass layer having an alkali metal oxide content of at most 2 wt % and athickness of at most 50 μm, preferably at most 40 μm, particularlypreferably at most 30 μm.

The object of the invention is furthermore achieved by a method forproducing a capacitor, comprising the following steps: providing a glasshaving an alkali metal oxide content of at most 2 wt %; drawing theglass into a glass strip having a fire-polished surface with a thicknessof at most 50 μm; assembling the glass strip with a first metal layerand at least one second metal layer to form a unit; and electricallycontacting the two metal layers.

The object of the invention is fully achieved in this way.

Specifically, according to the invention it has been found that when lowalkali metal or alkali metal-free glasses are used as dielectric, thedielectric breakdown strength of quartz glass can be achieved and evensignificantly surpassed.

Furthermore, the breakdown strength is improved considerably by thefire-polished surface.

The use of low alkali metal thin fire-polished glass layers therebyprovides a surprisingly simple and economical way of producingcapacitors with a high energy storage density.

In a preferred refinement of the invention, the glass layer has athickness of at least 5 μm, in particular at least 10 μm, in particularat least 15 μm.

It has been found that particularly in this thickness range, preferablywith 15-30 μm, a particularly smooth surface is obtained by the strongerdrawing of the low alkali metal glasses, which favours a particularlyhigh dielectric breakdown strength.

According to another embodiment of the invention, the capacitorcomprises a layer unit which consists of at least a first glass layer onwhich a first metal layer is provided, on which at least one secondglass layer and at least one second metal layer are provided, the layerunit being wound and the two metal layers respectively beingelectrically connected to terminals.

It has been found that such thin glass layers can also readily be wound,so that a particularly large amount of energy can be stored in a verysmall space.

In another preferred embodiment of the invention, each glass layerconsists of a glass having an alkali metal oxide content of at most 1 wt%, preferably at most 0.5 wt %, more preferably at most 0.1 wt % andparticularly preferably at most 0.05 wt %.

By using particularly low alkali metal glasses, the breakdown strengthcan be increased even further according to the invention.

According to another preferred embodiment of the invention, each glasslayer has a quality (ζ) of at least 20·10¹² V/mm³, preferably of atleast 50·10¹² V/mm², the quality being defined as the ratio of thebreakdown field strength to the product of the thickness and theroughness of the surface of the glass layer.

The smaller the thickness of the glass layer is, and the less theroughness of the glass surface is, the better is the breakdown strength,as expressed by this parameter.

When using a glass layer of high quality, it is possible to achieve aparticularly high energy density or a particularly small overall size.

According to another preferred embodiment of the invention, each glasslayer consists of a glass having an electrical conductivity of at most10⁻¹⁵ S/cm.

When using such glasses with a very low electrical conductivity, a timeconstant for the discharge of the order of minutes to days is obtained.This means that the discharge of the capacitor is determined not by theresidual dielectric conductivity but rather by the package of thecapacitor and further factors.

According to another configuration of the invention, the glasses usedhave a dielectric loss angle (tan δ) of at most 0.001 at 1 kHz.

Particularly suitable glasses for producing the dielectric have thefollowing components (in wt % based on oxide):

SiO₂ 40-75  Al₂O₃ 1-25 B₂O₃ 0-16 Alkaline-earth metal oxides 0-30 Alkalimetal oxides 0-2. 

In particular, glasses which contain the following components (in wt %based on oxide) are preferred:

SiO₂ 45-70  Al₂O₃ 5-25 B₂O₃ 1-16 Alkaline-earth metal oxides 1-30 Alkalimetal oxides 0-1. 

Preferably, the maximum mean roughness depth (RMS) is 1 nanometer,preferably at most 0.8 nanometer, particularly preferably at most 0.5nanometer.

With such a smooth surface, a particularly high dielectric breakdownvoltage is achieved. Typically, the maximum mean roughness depth isabout 0.4 nanometer for a fire-polished glass layer.

According to a preferred refinement of the method according to theinvention, a first and a second glass strip are drawn, and the firstglass strip is assembled together with the first metal layer, with thesecond glass strip and a second metal layer, and wound to form a unit.

It has been found that this allows particularly economical production.In this case, an adhesive layer (for example epoxy adhesive) may beapplied between neighbouring layers, for example as a sprayed layer, inorder to achieve a permanent bond. As an alternative, the adhesive layermay already be present as a coating on the wound metal layer (metalfoil). In this case, a sheet of paper or the like would also need to bewound in as a separating layer, which is then unrolled before bonding tothe glass layer.

As a further alternative, each glass strip may first be drawn from themelt and wound with paper to form a roll. Then, at a later time duringthe method, the metal layer may be applied or the glass strip may bewound together with a metal foil after unrolling the paper layer.

All conceivable metals which are suitable may be envisaged. Aluminium,for instance, has the advantage of being particularly inexpensive.Nickel layers have the advantage of diffusing less, but they are moreexpensive. Although the silver layers are even more expensive, theynevertheless have the advantage of particularly good electricalconductivity, although they lead to greater diffusion. It is to beunderstood that suitable alloys may naturally also be used.

According to the invention, production of each glass strip by thedown-draw method or by the overflow down-draw fusion method isparticularly preferred.

It has been found that both methods, which are widely known in the priorart (cf. for example WO 02/051757 A2 for the down-draw method and WO03/051783 A1 for the overflow down-draw fusion method) are particularlysuitable for drawing thin glass strips of 50 μm or less. Since bothmethods are fundamentally known, they will not be explained in furtherdetail here. For details, reference is made to WO 02/051757 A2 and to WO03/051783 A1, the disclosure of which is fully incorporated by referencehere.

In a preferred refinement of the invention, each glass strip isseparated into a plurality of individual strips by means of a lasercutting device after it has been drawn.

This simplifies the method sequence for the production of smallcapacitors starting from relatively wide glass strips. The use of alaser cutting method ensures very nondestructive separation of the glassstrip into smaller glass strips, which preferably also gives a surfacelike a fire-polished surface on the edges. In this way, a particularlyhigh dielectric breakdown voltage can be ensured.

According to another configuration of the invention, each glass strip isdrawn to an even smaller thickness by a down-draw method after it hasbeen drawn.

Since glass strips with a small thickness are particularly preferred,even thinner glass strips with an even further improved surface can thusbe produced according to the invention, so that the energy density ofthe capacitor produced can be increased further.

It is to be understood that all the features mentioned above and yet tobe explained below may be used not only in the combination respectivelyindicated, but also in other combinations or individually, withoutdeparting from the scope of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

Other features and advantages of the invention may be found in thefollowing description of preferred exemplary embodiments with referenceto the drawing, in which:

FIG. 1 shows a nozzle device for drawing a glass strip by the down-drawmethod;

FIG. 2 shows a greatly simplified representation of an apparatus forproducing and winding a unit comprising two glass layers and two metallayers, which are combined with one another in alternating sequence, thetwo glass layers respectively being produced by the down-draw method;

FIG. 3 shows a simple representation of an apparatus for drawing a glassstrip by the down-draw method with a subsequent laser cutting device forseparating the produced glass strip into a plurality of individual glassstrips;

FIG. 4 shows a schematic view of a capacitor according to the invention;

FIG. 5 shows the relationship between the breakdown field strength (inkV/mm) and thickness (mm); and

FIG. 6 shows the quality (ζ) (V/mm³) of the glass layer as a function ofthickness (mm).

DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1 shows a nozzle device 10 of fundamentally known design, which isused in a down-draw method in order to draw sheet glass very thinly withextremely high precision. In the down-draw method, which is generallydescribed in WO 02/051757 A2, bubble-free and highly homogenized glassflows into a glass reservoir, the so-called drawing tank. The drawingtank consists of noble metals, for instance platinum or platinum alloys.A nozzle device 10 comprising a slit nozzle 14 is arranged below thedrawing tank. The size and shape of this slit nozzle 14 define thethrough-flow of the glass strip 16 being drawn and its thicknessdistribution over the width of the glass strip. The glass strip is drawndownward by using drawing rollers 26, 28 (FIG. 2) and lastly passesthrough an annealing oven (not shown) which follows on from the drawingrollers. The annealing oven slowly cools the glass down to roomtemperature, in order to avoid stresses in the glass. The speed of thedrawing rollers defines the thickness of the glass strip. After thedrawing process, the glass is bent from the vertical position to ahorizontal position for further processing.

The glass strip 16 produced may, as is known in the prior art, be woundtogether with paper to form a roll. For further production of thecapacitor, the paper must then be unrolled again at a later time and inits place a metal foil, for example a thin aluminium foil, must bebrought in contact with the glass. In order to produce a capacitor, forinstance according to FIG. 4, a first glass strip is then wound togetherwith a metal foil, followed by a second glass strip, followed by asecond metal foil, to form a unit, and brought to the size of the endproduct. The two metal foils electrically insulated from one another bythe glass layer are then provided with terminals 72, 74, so that acapacitor 70 is obtained. The wound unit 48 according to FIG. 4 ispreferably also enclosed by a package 76, which may for example beproduced by melting a glass with a relatively low melting point aroundit.

FIG. 2 schematically represents an apparatus 20 for producing a unit 48,consisting of a first glass layer, followed by a metal foil, followed bya second glass layer, in turn followed by a second metal layer, andwinding it onto a roll 50. A first glass strip 16 emerges from a firstdrawing device 22 and is drawn by drawing rollers 26. After sufficientcooling, the glass strip 16 is wound together with a metal foil, forexample an aluminium foil 32, which is unwound from a roll 30 anddelivered via a guide roller 34. From a second drawing device 24, asecond glass strip 18 is drawn via drawing rollers 28 and issubsequently delivered via guide rollers 36, 38 onto the surface of themetal foil 32.

A second metal foil, for instance in the form of an aluminium foil 42,is subsequently unwound from a roll 40 and delivered via a guide roller44. The unit 48 formed in this way is optionally wound via further guiderollers 45 onto a roll 50. In this case, it is necessary to take carethat the first metal foil protrudes only on one side and the secondmetal foil only on the other side, in order to avoid arcing or shortcircuits.

Lastly, it can be seen from FIG. 3 that each glass strip may beseparated into a plurality of individual strips, which is preferablycarried out immediately after drawing the glass strip 16 on the drawingdevice 22. FIG. 3 schematically represents a glass strip 16 drawn fromthe drawing device 22. The representation of drawing rollers, coolingzones and the like has been omitted for the sake of clarity. Afterleaving the drawing device 22, the glass strip 16 is first automaticallychecked by means of an inspection device 60. This may, for example,involve a camera-based system. Subsequently, the glass strip 16 isdivided by means of a laser cutting device comprising a plurality oflasers 52, 54, 56, 58 into a series of individual strips 16 ^(I), 16^(II), 16 ^(III), 16 ^(IV), 16 ^(V). These individual strips 16 ^(I), 16^(II), 16 ^(III), 16 ^(IV), 16 ^(V), with a width of for example 10 cmeach, may be processed further in a way corresponding to that of FIG. 2,or may respectively be wound with paper.

EXAMPLES

Glass strips of the glass D263 with a thickness of 30 μm and glassstrips of the glass AF45 with a thickness of 50 μm were produced by thedown-draw method and each wound together with paper.

The suitability of the glass sheets produced for the production ofcapacitors was subsequently tested.

The typical composition of the two glasses AF45 and D263, both of whichare manufactured and marketed by Schott AG, is summarized in Table 1.

The glass AF45 is accordingly an alkali metal-free glass, which isparticularly preferred according to the invention. Conversely, the glassD263 is a comparative example since it contains 16 wt % of alkali metaloxides.

The mean roughness (RMS) according to DIN ISO 1302, also referred to asan arithmetic mean roughness value (R_(a)), for both glass strips liesbetween about 0.4 and 0.5 nm. The surface is thus extremely smooth. Thedielectric breakdown strengths of the two glass strips AF45 and D263were determined. A high breakdown strength of about 5 to 7 MV/cm wasfound for the glass sheet made of AF45 with a thickness of 50 μm.

TABLE 1 Composition (wt %) AF45 D263 SiO₂ 59 57 Al₂O₃ 10 6 B₂O₃ 15 9 BaO25 — ZnO — 7 TiO₂ — 5 Alkali metal — 16 oxides

For the comparative example made of D263, an average breakdown strengthof only about 4 MV/cm was found. This means that the dielectricbreakdown strength of the alkali metal-free glass AF45 is much betterthan that of the glass D263 containing alkali metal. In this context, itshould be taken into account that it is generally known that thebreakdown strength of a dielectric increases with a decreasing thickness(to a certain limit value). This therefore shows the significantlyimproved breakdown strength of the alkali metal-free glass AF45, since ahigher breakdown strength was measured despite the greater thickness.

The total energy density E is related to the field strength U/d asfollows:

${E = {\frac{1}{2} \cdot ɛ_{0} \cdot ɛ \cdot \left( \frac{U}{d} \right)^{2}}},$where ∈₀=8.854187·10⁻¹² As/Vm is the dielectric field constant, ∈ is therelative permittivity, U is the applied voltage and d is the thicknessof the dielectric.

The relative permittivity for AF45 was determined as 6.2. Assuming adielectric breakdown field strength of 7 MV/cm, an energy density of20·10⁶ Ws/m³ is obtained for the glass AF45, which corresponds to 3.7Wh/l.

TABLE 2 Composition (wt %) AF32 AF37 8252 SiO₂ 61 57 60 Al₂O₃ 18 17 14.5B₂O₃ 10 8 4.5 CaO 5 2 10 BaO 3 3 9 MgO 3 5 2 ZnO — — — TiO₂ — — — SrO —— — Alkali metal — — — oxides Remainder — 2 —

Assuming the expected higher dielectric breakdown field strength of 12MV/cm, an energy density of about 40·10⁶ Ws/m³ is obtained for a thinnerglass sheet for the glass AF45, which corresponds to 11 Wh/l.

For the glass D263 which contains about 16 wt % of alkali metal oxides,conversely, a dielectric breakdown strength of merely about 4 MV/cm wasdetermined, which leads to a correspondingly lower characteristic energydensity.

The significantly higher dielectric breakdown strength of the glass AF45compared with the glass D263 is attributed to the alkali metal freedomof the glass AF45.

It is assumed that other alkali metal-free glasses, for example AF32,AF37 and 8252, which are likewise manufactured and marketed bySchott-AG, lead to similarly good results. The compositions of theseglasses are indicated in Tab. 2.

Table 3 summarizes further alkali metal-free glasses which areparticularly suitable for the capacitor production according to theinvention.

TABLE 3 Composition (wt %) 1 2 3 4 SiO₂ 63 56 60 59 Al₂O₃ 17 11 17 15B₂O₃ 10 6 8 10 CaO 10 3 4 6 MgO — 2 4 — BaO — 15 — 4 SrO — 7 7 6

It is to be understood that the aforementioned glasses according to Tabs1 to 3 may contain up to about 0.5 wt % of alkali metal oxides asimpurities (owing to contamination of the raw materials and melting outfrom the refractory cladding).

FIG. 6 graphically represents the relationship between the breakdownfield strength (kV/mm) and the thickness of the glass layer (mm) for theglass AF45. It can be seen that the breakdown field strength increaseswith a smaller thickness.

FIG. 7 shows the quality of the glass layer (V/mm³) as a function of thesample thickness (mm) for the glass AF45, a value of 0.4 nanometer(typical of fire-polished surfaces) being assumed for the RMS roughness.It can be seen that particularly advantageous values lie above 20¹²V/mm³. Maximum energy densities are thereby achieved.

What is claimed is:
 1. A method of producing a capacitor, comprising:providing a glass being essentially free of alkali metal oxides; drawingsaid glass into a glass strip having a fire-polished surface with athickness of at most 50 μm; assembling said glass strip with a firstmetal layer and a second metal layer to form a unit; and electricallycontacting said first and second metal layers.
 2. The method of claim 1,wherein said drawing step comprises drawing a first and a second glassstrip, wherein said assembling step comprises assembling said firstglass strip together with said first metal layer and assembling saidsecond glass strip together with said second metal layer, and furthercomprising winding said assembled first and second glass strips and saidfirst and second metal layers to form a unit.
 3. The method of claim 2,further comprising placing an adhesive layer placed between said firstglass strip and said first metal layer and between said second glassstrip and said second metal layer.
 4. The method of claim 2, whereinsaid first and second glass strips are produced by a down-draw method oran overflow down-draw fusion method.
 5. The method of claim 1, furthercomprising separating said glass strip into said first and second glassstrips after drawing by laser cutting.
 6. The method of claim 1, furthercomprising further drawing said glass strip an even smaller thickness bya down-draw method after said drawing step.
 7. The method of claim 1,wherein said thickness of said glass strip is at least 5 μm.
 8. Themethod of claim 1, wherein said thickness of said glass strip is at most30 μm.
 9. The method of claim 1, wherein said glass comprises thefollowing components in weight percent based on oxide: SiO₂ 45-70; Al₂O₃ 5-25; B₂O₃  1-16; alkaline-earth metal oxides  1-30; and alkali metaloxides  0-1.


10. The method of claim 1, wherein said glass comprises the followingcomponents in weight percent based on oxide: SiO₂ 40-75, Al₂O₃  1-25,B₂O₃  0-16, alkaline-earth metal oxides  0-30, and alkali metal oxides 0-2.


11. The method of claim 1, wherein said glass has an electricalconductivity of at most 10⁻¹⁵ S/cm.
 12. The method of claim 1, whereinsaid glass has a dielectric loss angle of at most 0.001 at 1 kHz. 13.The method of claim 1, wherein the step assembling said glass strip witha first metal layer and said second metal layer to form the unitcomprises: winding said first and second metal layers and said glassstrip; and electrically connecting said first and second metal layers toterminals, respectively.
 14. The method of claim 1, wherein said glassstrip has a maximum mean roughness depth of at most 1 nanometer.
 15. Themethod of claim 1, wherein said glass has a quality of at least 20·10¹²V/mm³, said quality being defined as a ratio of a breakdown fieldstrength to a product of a thickness and a roughness of a surface ofsaid glass strip.
 16. The method of claim 1, further comprising lasercutting said glass to form a fire-polished surface on edges that havebeen laser cut.